Open door

Until last week I hadn't given much thought to the gigatonne. I suspect that many of you are in the same position. However, the launch of the Guardian's 10:10 campaign to reduce carbon emissions and an abundance of articles about matters such as melting glaciers, wind turbines and solar panels have put the spotlight on this and other, more familiar, units of measurement.

The Guardian's science correspondents are the kind of people who know their kilowatts from their kilowatt hours, but they don't, of course, write every story that includes scientific facts – two recent articles, written by others, mixed up the units. In one story, about solar panels, the contributor's words were changed from: "When the sun shone, about 1.5kW of power was generated" to: "When the sun shone, about 1.5kW of energy was generated". Another piece, in the Technology section, said an inventor's prototype for a home wind turbine produced the equivalent of 1kWh continuous power, instead of 1kW continuous power. "Once again we have confusion between power and energy," said one of the comments posted to it. "Oi Grauniad sort out yer units," said another.

Energy is measured (and priced) in kilowatt hours and power is measured in kilowatts. Over the years we've corrected this basic mistake several times and there's a pleasing explanation of the difference between the units in this correction from 2003: "The kilowatt is a measure of power. Energy is power multiplied by time, the kilowatt-hour being one measure of it. Therefore for us to price energy at 4.59p per kilowatt leaves something unsaid, and to express output as 1,300 kilowatts per hour is similar to measuring the distance from Edinburgh to London as 300 miles per hour."

Readers come from a variety of educational backgrounds. Those of us who are not scientists value useful explanations of scientific facts. It's good journalistic practice to illuminate more obscure scientific facts with familiar images, so that readers can visualise information, but it can be difficult to avoid cliches; football pitches, Texas and pinheads are used so often as size comparators they're practically alternative units of measurement.

The non-specialist writer who drops an image into a story to help the reader needs to be wary of introducing an error. Tony Cullen, from London, wrote to me last week about an erroneous comparison in an "otherwise excellent" story about the Sermilik fjord, which reported that the annual net loss of ice and water from the Greenland ice sheet is 300-400 gigatonnes and said this was equivalent to a billion elephants being dropped in the ocean. As Cullen pointed out, since a gigatonne is a billion tonnes and elephants don't weigh 300-400 tonnes, that comparison was neither accurate, nor useful. "Better, perhaps, to stick to measures which are more directly equivalent and compare volumes," he said. "Consider that a gigatonne of water is simply one cubic kilometre. So the ice sheet is losing 300-400 cubic kilometres annually."

The elephants-in-the-ocean analogy was a few words in parentheses in a long story about which there were no other complaints. Was it over the top to publish a correction about it? Writing in the Washington Post last week, columnist Michael Kinsley mocked newspapers for owning up to mistakes that might be viewed as relatively unimportant. He was scathing about the New York Times corrections column: "The facts it corrects are generally so bizarre or trivial and its tone so schoolmarmish that the effect is to make the whole pursuit of factual accuracy seem ridiculous," he said.

A newspaper that is open about its errors is an easy target for derision, but the more than 22,000 emails sent to my office last year convince me that readers take accuracy more seriously than Kinsley is willing to admit. Several have said that even small errors in a story make them less willing to trust the rest of it. "I particularly appreciate the fact that the Guardian carries corrections," a reader, who complained about numerical and other errors, said last year. "Something other papers appear either too arrogant or gutless to contemplate."